Diorganomagnesium compounds are well known for their usefulness in a wide variety of chemical reactions. As reagents, these compounds can be used for the reduction of ketones, the metalation of aromatic compounds and the alkylation of metal halides or oxides to the corresponding metal alkyls. As catalysts, diorganomagnesium compounds are useful in the dimerization and polymerization of olefins, see British Pat. No. 1,251,177, the polymerization of epoxides, see U.S. Pat. No. 3,444,102, and the preparation of telomers, see U.S. Pat. No. 3,742,077. While they perform many of the same types of functions performed by Grignard reagents, diorganomagnesium compounds, owing to differences in electronic and steric factors, are more reactive than Grignard reagents toward certain types of compounds. In general, see also U.S. Pat. Nos. 3,646,231 and 3,822,219.
The utility of diorganomagnesium compounds is lessened by the fact that many are either solids or highly viscous liquids and all are unstable upon exposure to moisture and air. This problem is generally overcome either by dissolving the compound in an inert hydrocarbon solvent or by solvating the compound and by handling under an inert atmosphere. Many diorganomagnesium compounds, particularly those with straight chain lower alkyl groups with a chain length of up to 4 carbon atoms, are insoluble by themselves in hydrocarbon solvents and thus require solubilizing agents which will form a soluble complex. Examples of such solubilizing agents are alkyllithium compounds, see U.S. Pat. No. 3,742,077, dialkyl zinc compounds, see U.S. Pat. No. 3,444,102, alkali metal hydrides, see U.S. Pat. No. 3,655,790, and organoaluminum compounds, see U.S. Pat. Nos. 3,737,393 and 3,028,219.
Solvation involves the use of an ether or an organic base molecule to associate directly with the magnesium atom, thus rendering a liquid-phase complex. The solvated form is undesirable, however, since solvation seriously inhibits the effectiveness of the compound, particularly when the compound is used as a Ziegler-type catalyst. The use of ether is particularly undesirable due to considerations of flammability and explosibility, and because it introduces soluble RMgX according to the Schlenk equilibrium EQU R.sub.2 Mg+MgX.sub.2 .revreaction.2RMgX
where R is alkyl and X is halogen.
Solubilization also serves to reduce the viscosity of reaction mixtures whose high viscosity would otherwise inhibit the progress of the reaction and cause difficulty in handling and transferring. This problem is only partially solved by the use of chloroaryl solvents to form low viscosity suspensions of the insoluble compounds, as described in U.S. Pat. No. 3,264,360.
In addition, the insolubility of the lower alkyl magnesium compounds makes preparation of them in a form free of undesirable halides difficult. In particular, the direct reaction of magnesium metal with an organic halide is disclosed in Glaze and Selman, Journal of Organometallic Chemistry, Vol. 5, p. 477 (1967), and W. N. Smith, Journal of Organometallic Chemistry, Vol. 64, p. 25 (1974). These articles deal with the preparation of diorganomagnesium compounds with straight chain alkyl groups of 5 carbon atoms and higher. Such compounds are soluble in hydrocarbon solvents and thus readily separable from the concurrently produced magnesium halide and unreacted magnesium. When lower straight chain alkyls are used in this process, the desired diorganomagnesium compound is formed but is insoluble and exists as a slurry in the solvent together with the magnesium halide and unreacted magnesium metal. Thus a solubilizing agent is required when this process is used to make lower alkyl diorganomagnesium compounds. The latter are particularly desirable as reagents and catalysts owing to their relatively high magnesium content on a weight basis.
Other methods of preparation include the mercury-magnesium exchange method, as disclosed in Cowan and Mosher, Journal of Organic Chemistry, Vol. 27, p. 1 (1962), and the dioxanate-precipitation method, as disclosed in Schlenk, Berichte der Deutschen Chemischen Gesselschaft, Vol. 64, p. 734 (1931). The mercury method, EQU R.sub. 2Hg+Mg.fwdarw.R.sub.2 Mg+Hg
where R is alkyl, is limited by the high cost of dialkylmercury compounds, and the health hazards involved in their use. The reaction itself is hazardous since it proceeds rapidly and exothermically after an inhibition period.
The dioxanate-precipitation method, EQU 2RMgX+C.sub.4 H.sub.8 O.sub.2 .sup.ether R.sub.2 Mg+C.sub.4 H.sub.8 O.sub.2.MgX.sub.2 .dwnarw.
where R is alkyl and X is halogen, involves removal of magnesium halide from ether solutions of Grignard reagents by precipitation of a complex which the dioxane forms with the halide. This is a tedious process and results in an etherated dialkylmagnesium complex from which the ether must be removed prior to use of the dialkylmagnesium as a catalyst.
Dialkylmagnesiums can also be prepared from alkyllithiums, see U.S. Pat. No. 3,646,231, by precipitation of lithium halide, EQU MgX.sub.2 +2RLi.fwdarw.R.sub.2 Mg+2LiX
where R is alkyl and X is halogen. This process is unsuitable for straight-chain lower alkyl diorganomagnesiums which are insoluble in hydrocarbon solvents, since separation of the diorganomagnesium from the product mixture is impossible. The use of basic solvents renders separation possible but requires subsequent desolvation.
Also disclosed in the same reference is the use of a hydrocarbon-soluble diorganomagnesium to solubilize an insoluble diorganomagnesium. The solubilizing members shown in this reference, however, invariably contain branched-chain alkyl groups. Such branched-chain diorganomagnesium compounds cannot be prepared by the Glaze and Selman method mentioned above. This fact is established in the work of Kamienski and Eastham, Journal of Organic Chemistry, Vol. 34, p. 1116 (1968). Thus, resort to the lithium halide precipitation method is required. The use of two or more individually insoluble straight-chain diorganomagnesium compounds to mutually solubilize each other has not been disclosed, particularly such compounds which can be prepared by the direct reaction between magnesium metal and alkyl halide.
The general insolubility of dialkylmagnesium compounds with straight-chain lower alkyl groups is thought to be due to intermolecular association resulting in the formation of a polymer-type macro-structure wherein each magnesium atom is tetrahedrally surrounded by four alkyl groups. Known methods of solubilizing these compounds presumably operate to break some of the intermolecular bonds and thereby break down the macro-structure into smaller units. Solvation or complexing as described above are thought to bring about this effect.
Dialkylmagnesium compounds containing either straight-chain alkyl groups of five carbon atoms or more or branched-chain alkyl groups of any length are also known to be effective as solubilizing agents. Similar to other solubilizing agents, these compounds are thought to bring about the solubilizing effect by breaking the intermolecular bonds of the polymer-type structure. With alkylmagnesium compounds, however, the effect is thought to occur by way of alkyl interchange and re-association, whereby the solubilizing alkyl groups exchange positions with some of the straight-chain lower alkyl groups. Polymerization is thus sterically hindered, either because the substituted groups are unwieldy for a tetrahedral fit around the magnesium atom, or because the groups have some inherent solubility of their own.
Thus, it is surprising that certain independently insoluble and presumably polymer-forming dialkylmagnesium compounds can be combined to form a hydrocarbon-soluble composition. Stated differently, it is surprising and unexpected that alkyl interchange between dimethylmagnesium, diethylmagnesium, and di-n-propylmagnesium is sufficient to break down the intermolecular bonds and render a soluble mixture. This theory of alkyl interchange is offered merely to show the unexpected nature of the composition of the present invention, and is intended neither to define nor to limit the invention in any manner.
It is therefore an object of the present invention to provide hydrocarbon-soluble diorganomagnesium compositions of high magnesium content.
A further object of the present invention is to provide a process by which hydrocarbon soluble diorganomagnesium compositions of high magnesium content can be prepared by the direct reaction of alkyl halides with magnesium.
A still further object of the present invention is to provide a means for solubilizing straight chain lower alkyl diorganomagnesium compounds in hydrocarbon solvents.
Further objects will be apparent from the following description.